1. Field of the Invention
The present invention is directed to an electric assist steering system, and, more particularly, to a method and apparatus for detecting a motor stall condition in an electric assist steering system.
2. Description of Related Art
Electric assist steering systems are well known in the art. In such electric assist steering systems, an electric assist motor, when energized, provides torque assist to aid the driver in turning steerable wheels of the vehicle. The electric assist motor is typically controlled in response to both steering torque applied to the vehicle steering wheel and measured vehicle speed. A controller monitors steering torque and controls a drive circuit to, in turn, control current applied to the electric assist motor. Such drive circuits typically include field effect transistors (FET""s) or other forms of solid state switches operatively coupled between the vehicle battery and the electric assist motor. Current is controlled by pulse width modulation of the FET""s or switches.
If steering torque is applied and the steerable wheels are held immovable, such as when the wheels are against a curb, the temperature of both the electric assist motor and the FET""s rise. Such a condition is referred to as a stall condition of the electric assist motor. If the stall condition lasts for an extended period, the motor and/or FET""s can overheat and fail.
U.S. Pat. No. 5,517,415 to Miller, which is assigned to TRW, Inc., discloses a stall detection apparatus for an electric assist steering system. A stall detector controls electrical current which is supplied to the electric motor in response to detecting the occurrence of a stall condition. This prevents damage to both the motor and drive circuitry.
U.S. Pat. No. 4,532,567 to Kade discloses an electric assist steering system which measures actual current through the motor. If the measured current exceeds a limit, the motor drive current is reduced.
U.S. Pat. No. 5,097,918 to Daido et al. discloses an electric assist steering system that includes an electric assist motor connected to a steering mechanism through an electromagnetic clutch. The system further includes a sensor for sensing rotational motion of the electric assist motor. When torque is sensed and the rotary sensor indicates that the motor is not rotating, the system disengages the electromagnetic clutch.
U.S. Pat. No. 4,878,004 to Shimizu discloses an electric assist steering system that includes an electric assist motor and a controller for controlling operation of the motor. The system further includes a current sensor which senses the magnitude of the actual current through the electric assist motor. An average value motor current is determined. The motor is controlled in response to the determined average current value.
The present invention is directed to an apparatus for detecting a stall condition of an electric assist motor in a vehicle steering system. The apparatus includes a switching circuit for providing electrical power to the electric motor of the vehicle steering system. The switching circuit has a temperature condition. A temperature sensor is coupled with the switching circuit and provides an output signal having a value indicative of the temperature condition of the switching circuit. A stall detector is responsive to the output signal of the temperature sensor. Specifically, the stall detector is operable to reduce power to the electric motor to a first level based on motor commutation and to a second level (i) based on motor commutation and (ii) upon determining the temperature condition of the switching circuit is greater than or equal to a first temperature threshold.
Another aspect of the present invention is directed to a method detecting a stall condition of an electric assist motor in a vehicle steering system. The method includes providing electric power to the electric assist motor of the vehicle steering system through a switching circuit. A temperature condition of the switching circuit of the system is monitored and an output signal indicative of the temperature condition is provided. The temperature condition of the switching circuit is determined in response to the output signal. Upon determining a stall condition, a stall detector is enabled to reduce power to the electric motor to first level through the switching circuit. Upon determining the temperature condition of the switching circuit is greater than or equal to a first temperature threshold during a stall condition, the stall detector reduces power to the electric motor to a second level which is different from the first level. Preferably, the second level of power reduction is functionally related to the temperature condition of the switching circuit.